1. Field of the Invention
The present invention relates to a method of estimating the characteristics of an electrochemical system of a battery that are not directly measurable.
2. Description of the Related Art
The electrochemical battery is one of the most critical components of a hybrid or electric vehicle. Smooth operation of the vehicle is based on a smart battery management system (BMS) whose purpose is to operate the battery with the best compromise between the various dynamic load levels. Precise and reliable knowledge of the state of charge (SoC), the state of health (SoH) and the thermal state (T) is essential for the BMS.
The SoC of a battery is the available capacity thereof (expressed as a percentage of its nominal capacity). Knowing the SoC allows estimation of how long the battery can continue to supply energy at a given current or how long it can absorb energy. This information conditions the operation of the entire vehicle and notably the management of the energy among its components.
During the life of a battery, its performances tend to degrade gradually due to the physical and chemical variations that occur during use, until the battery becomes unusable. The state of health (SoH), which is the available capacity after recharging (expressed in Ah), thus is a measurement of the point that has been reached in the life cycle of the battery.
The thermal state (T) of a battery conditions its performances because the chemical reactions and transport phenomena involved in the electrochemical systems are thermally activated. The initial thermal state is linked with the external temperature of the vehicle, which can operate within a wide temperature range, typically between −40° C. and +40° C. The thermal state during operation evolves depending on the battery draw under charge and discharge conditions, its design and its environment.
A more precise and reliable estimation of the SoC, the SoH and the thermal state T has several advantages. This estimation allows preventing the vehicle supervisor from functioning too conservatively regarding the use of the energy potential of the battery or inversely. It also allows avoiding safety oversizing of the battery and therefore saves on-board weight and, consequently, consumed fuel. It also allows reduction of the total cost of the vehicle. A correct estimator thus guarantees efficient and reliable use of the battery capacity over the entire operating range of the vehicle.
The SoC estimation method referred to as “Coulomb counting” or “book keeping” is known in the technical field of the invention, but covers estimation errors by disregarding phenomena such as self-discharge.
The no-load voltage measurement as a SoC indicator is also known. Using other indicators such as, for example, the estimation of an internal resistance is disclosed in U.S. Pat. No. 6,191,590 and EP Patent 1,835,297 A1.
The latter two methods are characterized by the fact that the SoC is first associated with one or more measurable or easily assessable quantities, through static mappings or analytical functional dependencies. However, these dependencies are in fact much more complicated than what is normally taken into account in the BMS, which often leads to SoC estimation errors.
A potentially more promising method is based on the measurement, by impedance spectroscopy (EIS), of a physical quantity parametrized by the SoC. For example, U.S. Published Patent Application 2007/0,090,843 suggests determination by EIS of the frequency f± associated with the capacitive/inductive transition. A correlation between frequency f± and the SoC is presented for a lead battery, for Ni—Cd batteries and Ni-MH batteries.
A similar approach is based on modelling the EIS spectra by equivalent electric circuits whose components are parametrized by the SoC, as described in U.S. Pat. No. 6,778,913 B2 filed by the Cadex Electronics Company, on which the development of an automotive battery tester Spectro CA-12 is based on the multi-frequency electrochemical impedance spectroscopy for the acid-lead pair. The EIS spectra are approximated by equivalent electric circuits and the evolution of the components is parametrized by the SoC. Similarly, U.S. Pat. No. 6,037,777 filed by K. S. Champlin determines the state of charge and other battery properties by measuring the real and imaginary parts of the complex impedance/admittance for lead batteries or other systems.
An alternative approach is based on mathematical battery models in order to use estimation techniques known in other fields. U.S. Published Patent Application 2007/0,035,307 notably describes a method of estimating the variables of state and the parameters of a battery from operating data (voltage U, current I, T), using a mathematical battery model. The mathematical model comprises a plurality of mathematical submodels which allow faster response. The submodels are models of equivalent electric circuit type, referred to as RC models, associated with restricted frequency ranges.
The use of RC models is also described in EP Patent 880,710 in which the description of the electrochemical and physical phenomena at the electrodes and in the electrolyte serve as a support for the development of the RC model. The temperature of the battery is simulated by the model in order to gain precision in relation to an external measurement.
In the models of RC type, the SoC is always introduced only to parametrize other variables. The SoC itself is never mentioned as an electrochemical variable.
Another SoC estimation method known in the literature is ([Gu, White, etc.]) which is based on the mathematical description of the reactions of an electrochemical system. The SoC is calculated from variables of state of the system. This description rests on charge, energy, material balances, . . . , and on semi-empirical correlations.